Well since the opening cinematic shows that the Traveler has now been found on Mars. Which leads me to believe that the humans were an accident. The cinematic shows that the traveler was making it rain on Mars, it wanted to make Mars habitable for life (which means making it more like earth). But the humans traveled to Mars for something, and found the Traveler. The Traveler saw this as an opportunity to just improve upon humans, instead of waiting years to make new life. So it followed us to earth, giving us new technology and reforming other planet's atmospheres. Until the collapse occurred, and the Traveler returned to our home planet to make it's last stand. Later it created the guardians and ghosts because it knew now that it could not wait any longer to create the it's army of light.